This invention relates to the fields of ethylene-vinyl ester interpolymers in particulate form and to processes for alcoholyzing such interpolymers to provide vinyl alcohol-containing interpolymers.
The term "ethylene-vinyl ester interpolymer particles" and terms of like import as used herein refer to polymers in particulate form containing interpolymerized ethylene and vinyl ester, including such polymers containing vinyl alcohol units and/or one or more other interpolymerized monomers, which in an untreated state exhibit tackiness and a tendency to coalesce in aqueous dispersions. In general, the tackiness and tendency toward coalescence of the ethylene-vinyl ester resins becomes particularly troublesome when the vinyl ester content of the resins is at least about 25%, these undesirable characteristics asserting themselves in greater degree with increasing amounts of vinyl ester. Tackiness is also influenced by the particle size distribution of the resins such that as the average size of the particles becomes smaller, tackiness increases.
The ethylene-vinyl ester interpolymer powders have a wide variety of applications such as flexibilizers and impact modifiers for other resins, notably, the polyvinyl chlorides, as substrate polymers for graft copolymerization with other monomers and as starting materials for the manufacture of ethylene-vinyl ester-vinyl alcohol terpolymers and ethylene-vinyl alcohol copolymers by alcoholysis. Frequently, an ethylene-vinyl ester interpolymer powder is obtained by the known technique of melt dispersion, i.e., by adding the interpolymer to water maintained at a temperature above the melting point of the resins, adding a suitable dispersing agent to the molten resin mixture, mechanically stirring the mixture to obtain an acceptable dispersion of interpolymer, cooling the aqueous dispersion to at least the solidification point of the molten dispersed interpolymer particles, filtering the solidified particles from the aqueous dispersion medium, and washing and drying the resin particles.
It is known from both U.S. Pat. Nos. 3,780,004 to Hoyt et al. and 3,510,463 to Bristol, and British Pat. No. 1,095,204, that particulate ethylene-vinyl ester interpolymers of relatively low vinyl ester content can be subjected to solid state alcoholysis. But due to the above-noted tendency of ethylene-vinyl ester interpolymers containing at least about 25% vinyl ester to coalesce in aqueous dispersions, attaining a practical rate of filtration of the particles is virtually impossible and the particles comprising the resulting filter cake will, upon drying, coalesce thereby excluding the use of the resin as a suitable source material for the aforementioned applications of ethylene-vinyl ester copolymers. Were it not for this tendency of ethylene-vinyl ester interpolymer particles to coalesce, a tendency which as noted becomes more pronounced with increasing vinyl ester content and diminishing particle size distribution, the direct use of the high surface area particulate filter cake for conversion to particulate vinyl alcohol-containing polymers by solid phase alcoholysis employing caustic alcohol would be economically and technically compelling.
While the use of nitrogen-containing organic materials as anti-blocking or slip agents for ethylene-vinyl ester resins is known, such substances are largely water insoluble solids which form a coating surrounding each resin particle. U.S. Pat. No. 3,474,063 to Powell proposes the use of N,N' dialiphatic thiodipropionamides to improve the anti-blocking and slip characteristics of ethylene-vinyl acetate copolymer. U.S. Pat. No. 3,474,058 to Ridgeway et al. accomplishes these goals employing a combination of a fatty acid salt and a fatty acid amide. In addition to a fatty amide, the ethylene-vinyl ester resins of U.S. Pat. No. 3,463,751 to Hasegawa also require the presence of polyvinyl chloride to exhibit non-blocking characteristics. U.S. Pat. No. 3,396,137 to Wharton discloses N-substituted unsaturated carboxylic amides, and U.S. Pat. No. 3,371,057 to Guttman discloses mixtures of crystalline paraffin wax and N,N-dioleylethylene-diamine, as slip agents for ethylenevinyl ester resins. The use of the foregoing materials to prevent coalescence of ethylene-vinyl ester interpolymer particles in aqueous dispersions is disadvantageous for several reasons. Since the materials are water insoluble, they cannot readily function in aqueous environments, at least without the additional presence of suitable dispersing agents. Moreover, since they cannot be removed by such a simple and inexpensive expedient as water washing, their recovery and reuse are problematical. Ethylene-vinyl ester interpolymer particles which are coated with water insoluble substances are entirely unsatisfactory starting materials for solid phase hydrolysis since the coatings may function as barriers to the caustic alcohol reagents employed in such hydrolysis procedures.
Accordingly, there has heretofore existed a need for an effective water soluble processing aid for ethylene-vinyl ester interpolymer particles contained in aqueous dispersion media, especially one which facilitates the filtration of the particles for subsequent conversion to vinyl-alcohol-containing resins by solid phase alcoholysis.